1. Field of the Invention
The present invention relates to a method for restoring the functionality of equipment subjected to heavy corrosion in a plant for the production of urea.
More specifically, the present invention relates to a method for repairing and restoring the functionality of metallic parts or equipment subjected to erosion and/or corrosion by contact, under conditions of high temperature and pressure, with fluids comprising water mixed with ammonia, urea and/or ammonium carbamate, typically present in a plant for the industrial production of urea.
2. Discussion of the Background
It is well-known that urea is obtained with industrial processes which require operating conditions of high temperature and pressure at least in some parts of the plant.
In these processes, ammonia, which is generally in excess, and carbon dioxide are reacted in one or more reactors, at pressures usually of between 100 and 250 bar and temperatures of between 150 and 240.degree. C., obtaining as end-product an aqueous solution containing urea, ammonium carbamate not transformed into urea and the excess ammmonia used in the synthesis. The above aqueous solution is purified of the ammonium carbamate contained therein by its decomposition in decomposers operating, successively, at decreasing pressures. In most of the present processes, the first of these decomposers operates at pressures which are basically the same as the synthesis pressure or slightly lower, and generally uses stripping agents to decompose the ammonium carbamate and at the same time remove the decomposition products. Stripping agents can be inert gases, or ammonia or carbon dioxide, or mixtures of inert gases with ammonia and/or carbon dioxide, and the stripping can possibly be carried out by also using the excess ammonia dissolved in the mixture coming from the reactor (self-stripping), without requiring therefore any external agent.
The decomposition products of ammonium carbamate (NH.sub.3 and CO.sub.2), together with the possible stripping agents, excluding the inert gases, are normally condensed in suitable condensers obtaining a liquid mixture comprising water, ammonia and ammonium carbamate, which is recycled to the synthesis reactor. In plants which are technologically more advanced, at least one condensation step is carried out at pressures more or less the same as those of the reactor or slightly lower.
As a reference, among the many in existence, patents U.S. Pat. No. 3,886,210, U.S. Pat. No. 4,314,077, U.S. Pat. No. 4,137,262 and European patent application publication No. 504,966, can be cited, which describe processes for the production of urea with the above characteristics. A wide range of processes mainly used for the production of urea is described in "Encyclopedia of Chemical Technology", 3rd Edition (1983), Vol. 23, pages 548-574, John Wiley & Sons Ed.
The most critical steps of the process are those in which the ammonium carbamate is at its highest concentration and temperature, and therefore, in the above processes, these steps coincide with the reactor and subsequent equipment for the decomposition (or stripping) and condensation of the ammonium carbamate operating in similar or almost similar conditions to those of the reactor. The problem to be solved in this equipment is that of corrosion and/or erosion caused by the ammonium carbamate, ammonia and carbon dioxide which behave as highly corrosive agents, especially in the presence of water, at the high temperatures and pressures necessary for the synthesis of urea.
This problem of corrosion has been faced with different solutions in existing industrial plants, and others have been proposed in literature. There are, in fact, numerous metals and alloys capable of resisting for sufficiently long periods to the potentially corrosive conditions which are created inside a reactor for the synthesis of urea. Among these lead, titanium, zirconium and several stainless steels such as, for example, AISI 316L steel (urea grade), steel INOX 25/22/2 Cr/Ni/Mo, austenite-ferrite special steels, etc. can be mentioned. However, for economical reasons, this type of equipment cannot be entirely made of these corrosion-resistant alloys or metals. Generally containers or columns are used in normal carbon steel, possibly multilayer, having a thickness varying from 40 to 350 mm, depending on the geometry and pressure to be sustained (pressure resistant body), of which the surface in contact with corrosive or erosive fluids is uniformly lined with an anticorrosive lining from 2 to 30 mm thick.
In particular, the reactor basically consists of a vertical "Vessel" with inlet of the reagents from below and discharge of the reaction mixture from above. The pressure-resistant body normally consists of a cylinder having a diameter of from 0.5 to 4 m, made with the multilayer or solid wall technique, having the two ends closed by caps adequately welded thereto. Inside the reactor, all parts subjected to corrosion are lined with an anticorrosive lining which can be, for example, of titanium, lead, zirconium, or preferably, stainless steels (urea grade) of the type mentioned above.
The subsequent carbamate decomposer, especially if it operates at the same pressure as the reactor, consists of a shell-and-tube exchanger. Also in this case the "pressure-resistant body" is made of normal carbon steel, whereas titanium or urea grade stainless steels are preferably used for the lining.
The gases leaving the decomposer are usually recondensed in a carbamate condenser, which is still in contact with a mixture similar to that of the decomposer (except for urea) and is therefore very corrosive. Also in this case the internal lining is preferably made up of the above special urea grade stainless steels.
In the above equipments or plant units, the anticorrosive lining is made by assembling numerous elements having a suitable resistance to corrosion, forming, at the end, a structure which is sealed off at the high operating pressure. The different connections and weldings carried out for this purpose frequently require special techniques depending on the geometry and type of parts to be connected.
Whereas stainless steel is weldable to the "pressure-resistant body" below made of carbon steel, but has a higher coefficient of thermal expansion which, during operation, favours crack formation along the welding line, titanium cannot be welded onto steel and in any case has similar problems of cracks in the welding as its expansion coefficient is considerably lower than carbon steel.
For this reason techniques are used which often require complex equipment and operating procedures. In certain cases the lining is effected by welding deposit instead of sheets welded to each other and onto the pressure body. In other cases, especially with materials which cannot be welded to each other, it is necessary to "explode" the lining onto the pressure body to be sure of obtaining a satisfactory hold.
In all the above equipment there are however a certain number of "weep-holes" to detect possible losses of anticorrosive lining.
A weep-hole normally consists of a small pipe of 8-15 mm in diameter made of corrosion-resistant material and is inserted into the pressure body until it reaches the point of contact between the latter and the metal or alloy corrosion-resistant lining. If there is a leakage in the lining, owing to the high pressure, the internal fluid, which is corrosive, immediately spreads into the interstitial zone between lining and pressure body and, if not detected, it would cause the rapid corrosion of the carbon steel of which this latter is made. The presence of weep-holes enables these leakage to be detected. For this purpose all the interstitial zones below the anti-corrosive lining must communicate with at least one weep-hole. The number of weep-holes is usually from 2 to 4 for each ferrule and therefore, for example, in a reactor there are normally from 30 to 60 weep-holes.
The above equipment also has at least one circular opening, generally in the upper part, called "man-hole", which permits access to operators and equipment for controls and small internal repairs. These opening usually have diameters of between 45 and 60 cm and at the most allow the passage of objects having these sizes.
In spite of the numerous precautions and constructive contrivances mentioned above, it still frequently happens that large areas of the internal lining of equipment operating at high or medium pressure in contact with aggressive process fluids, such as those, for example, used in a plant for the production of urea, undergo heavy and extended corrosion which rapidly causes the risk of perforating the lining with the consequent danger of catastrophic breakages, or at the least makes it necessary to stop the plant for repairs which at times take a considerable amount of time.
Overcoming these phenomena of corrosion involves problems which are not easy to solve. Very often it is necessary to substitute the damaged equipment (reactor, exchanger or condenser) with a new one, suffering extremely high costs both for stopping the plant and for the construction and installation of the new equipment. Attempts to repair the damaged part have always been considered impossible in practice when the damage is considerable, both because of the conviction of not being able to offer a sufficient guarantee of safety for the operation and also for the practical difficulty of effecting it. Indeed, every intervention on equipment or plant units, to avoid its partial dismantling, must be carried out through the man-hole mentioned above. It is not therefore possible to insert into the equipment the metal laminates or other objects whose dimensions do not allow them to pass through the man-hole.
Considering the fact that the corroded areas are frequently extended onto surfaces of 20-30 m.sup.2 or more, it is easy to understand that it is absolutely impossible to re-line the corroded area with new uniform and homogeneous lining.
Another widely diffused prejudice also concerned the problem of fixing the repair onto the previous lining. It was thought, indeed, that owing to the considerable deterioration of the metal in the corroded area, a repair involving welding and supporting zones directly in the area of interest, as well as at the edges where the previous lining still had trustworthy characteristics, was not reliable.
For this reason it was the general opinion that it was not possible to restore the functionality of lining having extended areas of corrosion, with the sole use of the man-hole.
For the many reasons mentioned above, it was generally thought unadvisable or in any case economically inconvenient to carry out operations for the repair and functional restoration of corroded lining of equipment in the section of high or medium pressure of a plant for the production of urea.
The solution normally recommended for these problems of corrosion was consequently to substitute the damaged equipment, even though this involved high costs and the necessity of interrupting the production line for relatively long periods.